Referring to FIG. 1, a typical data storage system 10 includes at least one rack 12 of storage devices or enclosures 14, 14′. The data storage system 10 can have fewer or more enclosures than those shown (internal or external to the rack 12). In general, the enclosure 14 is a storage device having a plurality of disk drive modules 18. Examples of storage devices include disk-array enclosures (DAE) and disk-array processor enclosures (DPE). A typical DAE includes a plurality of disk drive modules (e.g., fifteen), one or two link control cards (LCCs), and one or two power supplies. A typical DPE includes a plurality of disk drive modules (e.g., fifteen), one or two storage processors, one or two LCCs, and one or two power supplies. Each disk drive module 18 includes a carrier assembly that holds a disk drive and slides into a slot of the enclosure 14.
Each enclosure 14, 14′ implements redundancy with an “A” side and a “B” side. In enclosure 14, for example, each side has a link control card (LCC) 22, 22′ and a power supply (not shown). Reference numerals for the B side components are the same as corresponding components on the A side with the addition of a prime (′) designation. Each LCC 22, 22′ includes a primary communications port 26, 26′ and an expansion communications port 30, 30′. The enclosures 14, 14′ are connected to each other by cables 34, 34′ in a loop topology. Communication signals traverse the loop in one direction and pass from enclosure 14 to enclosure 14′, in a daisy-chain fashion, and then return from enclosure 14′ to enclosure 14. An enclosure receiving communication signals targeted for a different enclosure forwards those signals along the loop.
A common implementation of the loop is a Fibre Channel (FC) arbitrated loop. Fibre Channel is a computer communications protocol for communicating signals at a data rate of up to 2 Gbps. In general, the FC protocol provides an interface by which host processors (and servers) communicate with the enclosures 14, 14′ and with the disk drive modules 18 installed within the enclosures 14, 14′. For typical FC data storage systems, the disk drive modules 18 have FC disk drives.
FIG. 2 shows the enclosure 14 having an FC disk drive 50, the A-side FC LCC 22 and the B-side FC LCC 22′ of FIG. 1. The FC disk drive 50 has two ports (dual port); that is, communications with the disk drive 50 can occur over either port. The FC disk drive 50 is electrically connected to a midplane 54 by a midplane connector 58. The FC LCCs 22, 22′ are also electrically connected to the midplane 54. One FC LCC 22 is in electrical communication with the FC disk drive 50 by way of one port, and the other FC LCC 22′ is in electrical communication with the FC disk drive 50 by way of the other port.
Disk drive manufacturers have for many years produced another type of disk drive, called Advanced Technology Attachment (ATA) disk drives (also known as IDE disk drives), for the personal computer (PC) industry. To satisfy the high volumes and the small profit margins of PCs, the disk drive manufacturers have refined their processes to improve the reliability and storage capacity of the ATA disk drives and to reduce their cost. Typically, the cost of an ATA disk drive is two to three times lower and its storage capacity two times greater than that of an FC disk drive. Thus, the net price per storage capacity advantage for ATA disk drives can be four to six times that of FC disk drives. Consequently, ATA disk drives present an attractive adjunct to FC disk drives for storage system developers. Notwithstanding their advantages, the use of ATA disk drives in the type of data storage system described above requires some adaptations.